1. Field of the Invention
The invention relates to lasers and is particularly directed to a feedback control system for stabilizing the light frequency of a laser.
2. Description of the Prior Art
The laser has found wide application as a research tool for fundamental investigations into the properties of nature. The extra-ordinary spatial and temporal coherence of some types of gas laser beams in particular has enabled the laser to find wide application in the field of metrology, and as an element in the control of various high precision laser systems (R. L. Barger, J. B. West, and T. C. English, Appl. Phys. Letters 27, 31 (1975)).
It has long been known that gas lasers are capable of emitting single frequency light waves of phenomenal stability in frequency. For example, Jaseja, Javan, and Townes, Phys. Rev. Letters 10, 165, (1963), observed under virtually ideal laboratory conditions fluctuations of as little as a few tens of cycles per second in a light frequency of 3.times.10.sup.14 --i.e a stability of one part in 10.sup.13. Such stabilities correspond to fluctuations on the mirror-to-mirror separation of only 1/2000 of the diameter of a hydrogen atom, or 1/20,000,000 of a wavelength, and can obviously only be obtained in the complete absence of vibration and temperature fluctuations. On the other hand, the stabilities observed with say a typical HeNe laser under typical laboratory conditions are limited to about 1 part in 10.sup.6, and most such lasers put out within the Doppler gain profile several very closely spaced frequencies, differing usually by 500 to 800 MHz. These properties make such simple inexpensive lasers unsuited to many applications in metrology and related control systems.
The scientific and patent (A. D. White, I.E.E.E. J. Quantum Electronics QE-1, 349, (1965); M. I. D'Yakonov and S. A. Fridikhov, Soviet Physics USPEKHI 9, 837, (1967); S. J. BENNETT, R. E. Ward, and D. C. Wilson, Appl. Optics 12, 1406, (1973); R. H. Morris, J. B. Ferguson, and J. S. Warniak, Appl. Optics 14, 2808, (1975); N. Umeda, M. Tsukiji, and H. Takasaki, Appl. Optics 19, 442, (1980); T. G. Polanyi and I. Tobias, U.S. Pat. No. 3,453,557 (1969); M. L. Skolnick, U.S. Pat. No. 3,622,908 (1971); A. Le Floch, U.S. Pat. No. 3,649,930 (1972); G. M. Burgwald, W. M. Kruger, and D. L. Hammond, U.S. Pat. No. 3,771,066 (1973), U.S. Pat. No. 3,889,207 (1975)) literature contains many active stabilization schemes for attaining single frequency and/or frequency stable operation. These vary from the very simple scheme described, S. J. Bennett, R. E. Ward, and D. C. Wilson, Appl. Optics 12, 1406 (1973), and patented, S. J. Bennett, and D. C. Wilson, British Pat. No. 1,448,676 (1974), by Bennett, to the complicated and sophisticated systems more recently described by Hackel, Hackel, and Ezekiel, Metrologia 13, 141 (1977). Under typical laboratory conditions, the scheme of Bennett achieved a short-term stability of about 3 MHz and a long-term stability of +5 MHz, while the latter authors were able to reduce short term fluctuations down to the order of 20 to 60 kHz with long-term stabilities of the order of 1 part in 10.sup.13.
The techniques of the present invention, when used with the relatively simple transverse Zeeman laser, R. H. Morris, J. B. Ferguson, and J. S. Warnick, Appl. Optics 14, 2808 (1975), will produce short-term stabilities similar to those obtained by the latter authors, and long-term stabilities substantially better than those heretofore obtained in small relatively inexpensive actively controlled lasers. At the same time, great versatility in useful operating modes is obtained.
While in principle, the light frequency of a laser can be held constant to a phenomenal degree, the light frequency of typical laboratory lasers is found to undergo very much larger erratic short-term excursions as well as more gradual long-term excursions. The short-term excursions manifest themselves as a broadened frequency spread or linewidth about the central light frequency, and the long-term excursions are seen as drifts in the light frequency itself. The recent development of tunable dye lasers, in which the laser light frequency can be varied in a controlled manner, has made possible many new laser applications in high-resolution spectroscopy. Such applications call for very precise tuning and very narrow spectral linewidth of the laser output beam, which can be provided only by an extremely stable laser.
A laser operated under typical laboratory conditions is subjected to numerous sources of short and long term frequency fluctuations. The operating frequency of the laser is selected by a resonant chamber or cavity tuned to resonate at a frequency within the range for which the laser can sustain its lasing action. Minute changes in the resonant length of the cavity will produce corresponding changes in the light frequency. For example, a short-term frequency stability of one part in 10.sup.9, attainable by the most stable of the commercially available lasers, corresponds to a change in a 30-cm cavity of only 3.times.10.sup.-8 cm. Resonant length variations of this magnitude can easily be induced by ambient vibrations from nearby equipment, wind-induced motion of the laboratory and above all by temperature variations. For example such small changes in cavity length can be induced by temperature fluctuations of the order of 1/50,000 part of a degree, such as can be generated by micro-turbulence resulting from heating-cooling fluctuations of the resonant cavity. In addition, there are other sources of bothersome frequency jitter which are not directly related to resonant-length variations, such as plasma disturbances within the cavity generated by fluctuations in the laser discharge current density, and partition noise generated by the stochastic distribution of decay pathways. Finally, there is the problem of retro-reflection, of which more is said at the conclusion of this introduction.
Various methods have been devised to reduce the frequency excursions of a laser output. In the passive approach, the laser is typically supported on a massive shock-mounted table and operated under stringently controlled laboratory conditions, usually in an isolated location, to eliminate as much as possible all sources of ambient vibration, sound, electrical current variations and especially temperature variations. Although this approach can provide remarkable short-term and long-term laser stability, it has the obvious drawback of requiring elaborate laboratory preparations impractical for most laser applications.
A common approach to obtaining laser stability is to direct a small part of the laser beam to a reference cavity or to an absorption cell and to use a a feedback servo-system to actively adjust the optical path length of the laser resonant cavity to correspond with optimum transmission or absorption, respectively.
In another common approach a feedback servosystem actively adjusts the optical path length of the laser resonant cavity, hence the resonant frequency, in response to a correction signal derived from the laser output radiation by means of synchronous detection. For example, in the scheme of U.S. Pat. No. 3,649,930 illustrating this approach, an alternating magnetic field is imposed on the laser in such a manner as to produce a modulation of the output light intensity. Variations of the modulated light beam are compared with the corresponding variations of the applied magnetic field inducing the modulation in the first instance by means of a synchronous detector, and the output signal therefrom is applied to a piezoelectric element controlling the position of one of the laser cavity mirrors.
A serious drawback inherent in any such synchronous-detection scheme is the inability to control frequency excursions occurring over a time interval less than the period of the applied modulating signal. The advantages of higher-frequency applied modulation can be lost, however, unless other delays in the system can be correspondingly reduced, most notably, the delay inherent in the transducer for adjusting the resonant optical path length of the laser. Moreover, the greatest stabilities achieved by this method are typically at a cost restriction to commercial profitability.
Another source of frequency shift of significance in virtually all active stabilization schemes is the reflection and scattering of the laser's own light back into the laser cavity--a phenomenon referred to herein as retro-reflection. Retro-reflected light causes the operating frequency of the laser to shift by an amount that depends on the amplitude and phase of the light re-entering the laser cavity. Retro-reflection must be eliminated to achieve an ultra-stable light frequency. Such retro-reflection caused by an object in the path of the laser beam can be suppressed by optically isolating that object. However, the difficulty lies in detecting the presence of retro-reflection in the first instance and in locating the particular objects causing the retro-reflection.